Glycosidase inhibition, important in the quest for highly potent and specific drugs, can be achieved by mimicking the oxocarbenium ion-like transition-state species that form during the catalytic mechanism. Castanospermine and calystegine B2 are potent inhibitors that are conformationally restricted by the inclusion of ethylene linkers. Their binding to a β-glucosidase from Thermotoga maritima has been studied by structural, kinetic and thermodynamic methods. Although both compounds inhibit with a similar potency, castanospermine derives the majority of its energetic contribution from enthalpy whereas calystegine B2 binding is more entropically driven.